3 Answers
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Besides the disproportionate ratio of followers to followings already mentioned, I've seen spammy followers show up because their area of work (?) is related to a keyword of my last tweet. I don't tweet often, and I don't have lots of followers, so this is pretty easy to spot manually.

For example, I whine in a tweet about getting CVs that contain "Java programmer" only to find out during the interview that they know little about Java. Then days/weeks later I have a new follower who specializes in recruiting Java programmers for IT companies.

My conjecture is that some spammers search tweets containing particular keywords, and then follow those tweeters, perhaps hoping to get a follow back and a voluntary reader of their spams.

When you get a new follower, cross reference the content of your last tweets and the keywords of their interests (if they're commercial). You'll detect the spammy followers.

An extreme following > followers ratio. An account that is following hundreds of people, but has only a handful of followers, is in many cases a spam account.

BUT by no means is this a straight indicator, merely a good warning sign to keep an eye out.

Obviously, 'people' you don't know personally sending you very short direct messages with a shortened url link are suspect (have had plenty of those, which you can easily block from then on).

Edit: just stumbled on this service today via Lifehacker: Fruji. The basic features are free to use, among which an analysis that determines Twitter accounts with a high 'marketing' factor, or put differently: whose primary aim is to build a marketing network.